This invention relates to a method of producing a closed end sleeve or a sleeve having a closed end, and more particularly to a core structure to be used in the method.
In order to produce closed end sleeves, a press-forming method has been used. However, a long closed end sleeve cannot be produced by the method. It is possible to produce such a long sleeve by attaching an end portion and a sleeve portion of the sleeve separately formed. In this case, however, such an attached sleeve is poor in strength at the attached portion of the sleeve. In order to eliminate this disadvantage of the method, the assignee of this application has proposed in Japanese Patent Application Laid-open No. 62-85,906, a method of producing a closed end sleeve having a closed end by extruding.
FIGS. 1a-1d illustrate the producing processes of the proposed method. As shown in FIG. 1a, a core member 23 having a paraffin tip 22 for forming an inner surface of an end or bottom of the sleeve to be produced is arranged in a mouthpiece 21, and an outer die 25 formed at its center with a through-aperture 24 for forming an outer surface of the end or bottom of the sleeve is arranged in a front end of the mouthpiece 21 to form an annular space by the mouthpiece 21, the core member 23 and the outer die 25. Thereafter, as shown in FIG. 1b, a forming material, for example, a ceramic material for forming the sleeve is supplied into the annular space and slightly extruded. When a minor amount of the ceramic material is extended from the through-aperture 24, the extruding operation is stopped. During this extrusion, the paraffin tip 22 is held in contact with the core member 23 via suction within the core member.
After the outer die 25 has been removed from the mouthpiece 21 as shown in FIG. 1c, the air is supplied into the core member 23 so as to extrude the ceramic material to obtain a cylinder or sleeve having a desired length as shown in FIG. 1d. The remaining paraffin tip at the end or bottom of the cylinder is removed when the cylinder is burned.
In the method of the Japanese Patent Application Laid-open No. 62-85,906, the paraffin tip 22 is used in order to remove the tip used for forming the end of the sleeve when burning the sleeve. However, the paraffin tip 22 is often collapsed by the forming pressure during the extruding process during the formation of the end of the sleeve or before the removal of the outer die 25. As a result, the inner configuration of the end or bottom of the sleeve is deviated from the required configuration.